1. Technical Field
The present disclosure relates to methods for the synthesis of N-substituted pyridinium compounds. The present disclosure also relates to novel purine-substituted pyridyl compounds which can be used in the methods and reactions disclosed herein.
2. Description of the Related Art
Pyridinium compounds are of interest in, for example, drug design and as general intermediates for organic syntheses, such as in natural product synthesis. Certain substituted pyridinium compounds are useful in the synthesis of NAD or NAD analogs, respectively. Additionally, certain heteroaryl substituted pyridinium compounds have been studied in relation to solvatochromism.
Triazinyl reactive dyes having carboxypyridine as a leaving group are known as useful in the coloration of cotton. In the coloration procedure, the pyridinium moiety acts as a leaving group when a triazinyl dye is reacted with nucleophiles. Fixation of the dye would not occur, however, if pyridinium ring opening (as in the Zincke-like pathway described herein) occurred.
Zincke salts may be prepared by reacting a pyridinium compound with 2,4 dinitro halobenzol, for example with 2,4 dinitrochlorbenzol and 2,4 dinitrobrombenzol.
Currently, the standard method for the synthetic production of substituted pyridinium compounds includes alkylation of pyridine derivatives. However, this reaction is only convenient when using primary alkyl halides. When secondary or tertiary alkyl halides are used, elimination occurs as an unwanted side reaction and yields are generally low. Moreover, when the alkylation is performed with alkyl halides having a halogen atom attached to an asymmetric carbon atom, racemization may occur during the nucleophilic displacement reaction.
As is apparent from the above description, the presently used activation reagents in the production of substituted pyridinium compounds are toxic, explosive, and/or otherwise hazardous, and are therefore limited to small scale research applications. Additionally, while there were sporadic attempts at performing Zincke reactions in an ecofriendly manner, for example by using microwave assisted synthesis, such attempts still rely on explosive dinitrophenyl compounds and are not capable of being scaled up without taking expensive precautionary measures.
As such, there is a need for an improved method for synthesis of N-substituted pyridinium compounds which avoids hazardous activation reagents and other problems known in the art.